The invention is directed to mixtures based on rubbery copolymers of ethylene, at least one other .alpha.-alkene, and, optionally one or more polyenes, characterized by good tack values and the process for preparing those mixtures.
Copolymers of ethylene with at least another .alpha.-alkene or ethylene copolymers containing a second .alpha.-alkene are popular synthetic rubbery polymers because of their excellent resistance to ozonization, weathering and aging. The resistance of these polymers to ozone, weathering and aging render these polymers attractive for various purposes.
However, conventional ethylene-.alpha.-alkene and ethylene-.alpha.-alkene-polyene elastomers, just as styrene-butadiene copolymers for example, are deficient with respect to tack properties and properties of adhesion.
Tack and adhesion, as referred to herein, connote the adhesion of unvulcanized rubber to itself, that is, connote the adhesion of two surfaces of two strips of unvulcanized rubber of identical composition when those two pieces are contacted with each other under moderate pressure.
Good tack properties of elastomers are essential, in the manufacture of various articles, and in particular multipartite or multilayered articles, such as, for instance, driving belts, conveyor belts, and, more in particular, car tires. Adequate tack properties will result in thorough adhesion of the various parts or layers of an article, each of which parts or layers are formed of the same rubber, during assembly and vulcanization.
The prior art has examined various alternatives for overcoming the inadequate tack properties of rubber copolymers based on ethylene-.alpha.-alkene and ethylene-.alpha.-alkene-polyene copolymers: for instance, one art recognized method includes the concept of adhering separate rubber surfaces to sandwich structures by means of adhesives applied in layers. Not only are such processes cumbersome, and hence unattractive for industrial purposes, but also such processes involve a certain amount of danger: inflammable organic solvents are often used in formulation and application of adhesives in those processes, so that those processes are not recommended, if only for the dangers inherent therein when undertaken on industrial scale.
In addition, the prior art has developed tackifier resins, e.g., condensation products of phenols and aldehydes and condensation products of acetylene, alkylated phenol resins including those sold under the trade name AMBEROL ST 140 F, phenol-substituted isoprene resins and phenol-substituted isoprene-isocopolymer resins. Use of tackifying resins provides the advantage that tackifying resins can be admixed with rubber to provide specific formulations and need not be applied to rubber surfaces in layers. Practically, it has been discovered that the actual amounts of tackifier resins necessary to provide reasonable tack levels in the rubbers admixed therewith impair other properties of the vulcanized rubber product. Moreover, the necessity of large amounts of tackifier resins results in migration of tackifier resins to the rubber surface in a phenomenon referred to as exudation. The manifestation of exudation is manifestly aggravated when rubber ethylene copolymers are rolled or calendered, since the exudation may result in complete negation of the effect desired by the inclusion of the aforementioned types of tackifiers and actually produce adverse effects. Furthermore, the use of large quantities of tackifiers is very expensive. Although tackifiers may be used in smaller amounts in which the adverse effects are obviated, the use of small amounts has failed to result in rubber formulations of satisfactory adhesion and tack.
United States patent application, Ser. No. 517,052, filed Oct. 22, 1974 is directed to a process for preparing rubbery copolymers based on polyethylene, of improved tack properties, in which the rubbery ethylene copolymer is formed of ethylene, at least one other .alpha.-alkene, one or more polyenes, in combination with one or more tackifying resins, in which is employed a copolymer of ethylene which contains carbon-carbon double bonds in the non-linear part of the polymer chain, the carbon atoms of which have either two vicinal hydrocarbon groups in a cis position with respect to each other but which do not form a cyclic system or have at least three hydrocarbon substituents; exposing a mixture of the copolymer, a tackifying resin and any optional conventional additives, in the presence of oxygen and photosensitizers, to an artificial light source emitting light at a wave length of 200-800 nanometers, the mixture of the copolymer, tackifying resin and optional conventional additives has an elongation at rupture of at least 500% and a tensile strength of between 1.5 and 50 kg/cm.sup.2 in the unvulcanized state.
Preferably in the said process the rubbery copolymer has a thermogram recorded at a cooling rate of 8.degree. C. per minute, showing a heat of crystallization as a function of the temperature having a peak at a temperature between -7.degree. C. and 11.degree. C.